Electroluminescent (EL) devices have been known for some years and have been recently used in commercial display devices. Such devices employ both active-matrix and passive-matrix control schemes and can employ a plurality of subpixels. In an active-matrix control scheme, each subpixel includes an EL emitter and a drive transistor for driving current through the EL emitter. The subpixels are typically arranged in two-dimensional arrays with a row and a column address for each subpixel, and having a data value associated with the subpixel. Subpixels of different colors, such as red, green, blue, and white are grouped to form pixels. Active-matrix EL displays can be made from various emitter technologies, including coatable-inorganic light-emitting diode, quantum-dot, and organic light-emitting diode (OLED), and various backplane technologies, including amorphous silicon (a-Si), zinc oxide, and low-temperature polysilicon (LTPS).
Some transistor technologies, such as LTPS, can produce drive transistors that have varying mobilities and threshold voltages across the surface of a display (Kuo, Yue, ed. Thin Film Transistors: Materials and Processes, vol. 2: Polycrystalline Thin Film Transistors. Boston: Kluwer Academic Publishers, 2004. pg. 410-412). This produces objectionable nonuniformity. These nonuniformities are present at the time the display is sold to an end user, and so are termed initial nonuniformities, or “mura.” FIG. 8 shows an example histogram of subpixel luminance exhibiting differences in characteristics between subpixels. All subpixels were driven at the same level, so should have had the same luminance. As FIG. 8 shows, the resulting luminances varied by 20 percent in either direction. This results in unacceptable display performance.
It is known to compensate for drive-transistor-related mura by employing a digital drive, or pulse-width modulated, display scheme. Unlike an analog drive display, in which the rows of the display are scanned sequentially once per frame period, a digital drive display scans the rows multiple times per frame. Each time a row is selected in a digital drive scheme, each subpixel in the row is either activated to output light at a selected level, or inactivated to emit no light. This is different from an analog drive display, in which each subpixel is caused to emit light at one of a plurality of levels corresponding to the available code values (e.g. 256).
For example, Ouchi et al., in U.S. Pat. Nos. 6,724,377 and 6,885,385, teach dividing each frame into a plurality of smaller subframes. This subframe configuration is controlled by a plurality of shift registers that activate the rows of pixel circuits in a plurality of interleaved sequences for data writing.
Kawabe, in commonly-assigned U.S. Patent Application No. 2008/088561, teaches an improvement to the above method wherein a single shift register is used to track the multiple sequences for data writing, and a series of enable control lines are used to control which of the multiple sequences is written at a given time. This method uses a two-transistor, one-capacitor (2T1C) subpixel circuit.
However, transistor-related mura is not the only cause of nonuniformity in an EL display. For example, as an OLED display is used, organic light-emitting materials in the display age and become less efficient at emitting light. Aging of an OLED emitter causes a decrease in the efficiency of the emitter, the amount of light output per unit current, and an increase in the impedance of the emitter, and thus its voltage at a given current. Both effects reduce the lifetime of the display. The differing organic materials can age at different rates, causing differential color aging and a display whose white point varies as the display is used. In addition, each individual subpixel can age at a rate different from other subpixels, resulting in display nonuniformity. Furthermore, changes in the temperature of an OLED emitter can change its voltage at a given current.
It is known to combine OLED emitters with low-temperature polysilicon drive transistors. In this configuration, the increase in OLED voltage as the emitter ages reduces the voltage across the drive transistor, and thus the amount of current produced. This causes further display nonuniformity.
One technique to compensate for these aging effects is described by Mikami et al. in U.S. Patent Application Publication No. 2002/0140659. This technique teaches a comparator in each subpixel to compare a data voltage to a rising reference voltage, or a falling data voltage to a fixed reference voltage. A data voltage is thus converted to an on-time of the EL subpixel. However, this technique requires complimentary logic or resistors on the EL display, both of which are difficult to fabricate on modern displays. Furthermore, this technique does not recognize the problem of OLED voltage rise or efficiency loss.
Kimura, in U.S. Pat. No. 7,138,967, describes using a current source and switch in every subpixel to drive uniform current during the on-time. This mitigates elevated black levels, a common problem with current-mode drive, but requires a very complex subpixel circuit which can reduce the aperture ratio, the amount of light-emitting area available in the subpixel. This requires an increase in current density through the EL emitter to maintain a given luminance, accelerating the very aging for which the technique intends to compensate.
Yamashita, in U.S. Patent Application Publication No. 2006/0022305, describes a six-transistor, two-capacitor subpixel circuit driven in a scan phase, a light emission phase, and a reset phase during which the threshold voltage of the drive transistor and the turn-on voltage of the OLED are stored on capacitors connected to the data voltage terminal. This method does not compensate for OLED efficiency loss, and it requires a very complex subpixel having a very small aperture ratio. Such a subpixel ages more quickly and has lower manufacturing yields.
U.S. Patent Application Publication No. 2002/0167474 by Everitt describes a pulse width modulation driver for an OLED display. One embodiment of a video display includes a voltage driver for providing a selected voltage to drive an organic light-emitting diode in a video display. The voltage driver can receive voltage information from a correction table that accounts for aging, column resistance, row resistance, and other diode characteristics. In one embodiment of the invention, the correction tables are calculated prior to or during normal circuit operation. Since the OLED output light level is assumed to be linear with respect to OLED current, the correction scheme is based on sending a known current through the OLED diode for a duration sufficiently long to permit the transients to settle out, and then measuring the corresponding voltage with an analog-to-digital converter (A/D) residing on the column driver. A calibration current source and the A/D can be switched to any column through a switching matrix. However, this technique is only applicable to passive-matrix displays, not to the higher-performance active-matrix displays which are commonly employed. Further, this technique does not include any correction for changes in OLED emitters as they age, such as OLED efficiency loss.
Arnold et al., in U.S. Pat. No. 6,995,519, teach a method of compensating for aging of an OLED device (emitter). This method relies on the drive transistor to drive current through the OLED emitter. However, drive transistors known in the art have non-idealities that are confounded with the OLED emitter aging in this method. Low-temperature polysilicon (LTPS) transistors can have nonuniform threshold voltages and mobilities across the surface of a display, and amorphous silicon (a-Si)transistors have a threshold voltage which changes with use. The method of Arnold et al. will therefore not provide complete compensation for OLED efficiency losses in circuits wherein transistors show such effects. Additionally, when methods such as reverse bias are used to mitigate a-Si transistor threshold voltage shifts, compensation of OLED efficiency loss can become unreliable without appropriate and expensive tracking and prediction of reverse bias effects.
Naugler et al., in U.S. Patent Application Publication No. 2008/0048951, teach measuring the current through an OLED emitter at various gate voltages of a drive transistor to locate a point on precalculated lookup tables used for compensation. However, this method requires a large number of lookup tables, consuming a significant amount of memory.
There is a need, therefore, for a more complete compensation approach for electroluminescent displays.